Process and apparatus for the separation of light-boiling components from hydrocarbon mixtures

ABSTRACT

A process and apparatus for the separation of a feed mixture into higher-boiling and lower boiling fractions in a continuously operated distillation apparatus uses at least one inlet pipe for feeding feed mixtures, an outlet for the lower-boiling fraction, an outlet for the higher-boiling fraction and a heating device. The distillation apparatus includes at least two condensation stages, each of a different temperature level, the condensation stages upstream in direction of the vapour flow having a higher temperature level than the downstream stages. Separation-effective internals are installed between the condensation stages, partial condensation taking place in the condensation stages, partial amounts that have not condensed in these stages being fed to downstream separation-effective internals or condensation stages of lower temperature level and partial amounts that have condensed are passed via separation-effective internals in direction of the outlet means for the higher-boiling fraction. A basically vaporous fluid is obtained at the condensation stage of the lowest temperature level, where it partially condenses, the uncondensed part of the fluid being fed to the outlet for the lower-boiling fraction and the condensed part being passed to a section of the distillation apparatus which is upstream of the condensation stage having the lowest temperature level. The condensation stage of the lowest temperature level having a temperature of below −40 ° C.

The invention relates to a process and an apparatus for the separation of light-boiling components from a hydrocarbon stream, particularly for the separation of a C₂ fraction from a C₃+ fraction to be obtained as target product, e.g. in the dehydrogenation of light hydrocarbons, but also suitable for other separation tasks within the range of C₁ to C₄.

Usually, such separations are performed by means of a so-called “coldbox” in combination with a distillation column. The distillation column is referred to and operated as a de-ethanizer because all substances of boiling points less or equal to that of ethane are separated overhead by this de-ethanizer.

Before entering the coldbox, the feed mixture is cooled down to approx. −25° C. The condensate gained is passed directly into the de-ethanizer. Vapours that have not condensed are further cooled to approx. −90° C. in the coldbox, the product-enriched condensate thus obtained also being directed to the de-ethanizer after heat exchange. The coldbox therefore performs a one-step rough separation.

The remaining vapour phase basically comprising uncondensable components, e.g. hydrogen, is depressurised after heat exchange in the flow entering the coldbox. The light-boiling substances cool down to approx. −110° C. due to the Joule-Thompson effect. This temperature level is used to partially condense the stream entering the coldbox. The uncondensed light-boiling substances are basically free from C₃+ components.

Finally all product-enriched condensate phases are led to a final separation in the de-ethanizer where the remaining light-boiling components are separated form the heavier boiling components. For this purpose a temperature of about −20° C. is necessary at the head of the column.

Evaporating propane or propene can be used at −30° C. as a cooling agent for cooling the feed mixture before it enters the coldbox, for operating the coldbox and for cooling the de-ethanizer.

The generation of cold for such a process is extremely expensive. The aim of the invention is, therefore, to provide a process and an apparatus by which the consumption of cold can be reduced to a significant degree.

The aim of the invention is achieved according to the main claim by a process for the separation of a feed mixture

-   into a higher-boiling and a lower boiling fraction in a continuously     operated distillation apparatus comprising at least one inlet pipe     for feeding one or more feed mixtures, an outlet means for the     lower-boiling fraction, an outlet means for the higher-boiling     fraction and a heating device, in which -   the distillation apparatus includes at least two condensation     stages, each of a different temperature level, -   the condensation stages provided upstream in direction of the vapour     flow having a higher temperature level than the downstream     condensation stages, -   separation-effective internals are installed between the     condensation stages, -   partial condensation takes place in the condensation stages, -   partial amounts that have not condensed in these stages are fed to     downstream separation-effective internals or condensation stages of     lower temperature level and partial amounts that have condensed are     passed via separation-effective internals in direction of the outlet     means for the higher-boiling fraction, -   a basically vaporous fluid is obtained at the condensation stage of     the lowest temperature level, where it partially condenses, -   the uncondensed part of the fluid being recycled to the outlet means     for the lower-boiling fraction and the condensed part being recycled     to a section of the distillation apparatus which is upstream of the     condensation stage having the lowest temperature level, -   and the condensation stage of the lowest temperature level having a     temperature of below −40° C.

In embodiments of the invention, the distillation apparatus comprises three to five successive condensation stages, each of a different temperature level.

In a further embodiment of the invention the condensation stage of the lowest temperature level is operated at a temperature between −120° C. and −70° C. and a pressure of at least 2 MPa absolute, preferably at least 3 MPa.

In a further embodiment of the invention the mixture which leaves the distillation apparatus as lower-boiling fraction is depressurised, the mixture thereby cooling down further under exploitation of the Joule-Thompson-effect, and thus being used to cool the condensation stage with the lowest temperature level.

In a further embodiment of the invention the depressurisation is performed by an expansion turbine.

Further embodiments of the invention relate to the use of suitable feed mixtures which are of particular advantage to the process for the purpose of obtaining useful products.

In a further embodiment of the invention the process is used for feed mixtures basically containing hydrogen, hydrocarbons of up to two carbon atoms and hydrocarbons of at least three carbon atoms. At the outlet means for the lower-boiling fraction a mixture is obtained which basically contains hydrogen and hydrocarbons of up to two carbon atoms and basically no hydrocarbons of at least three carbon atoms. At the outlet means for the higher-boiling fraction a mixture is obtained, which basically contains hydrocarbons of at least three carbon atoms and basically neither hydrogen nor hydrocarbons of up to two carbon atoms.

In a further embodiment of the invention the process is used for feed mixtures containing less than 2 mol-% each of carbon dioxide and water or water vapour.

In a further embodiment of the invention the feed mixture used is a reaction mixture from the catalytic dehydrogenation of hydrocarbons.

In a further embodiment of the invention the segment of the distillation apparatus, through which that part of the feed mixture that has condensed in the condensation stage of the highest temperature level is passed to the outlet means for the higher-boiling fraction, is provided as a stripping section of the distillation apparatus.

in further embodiments of the invention feed mixtures of a relatively small content of components of low boiling point are preferably fed at a point below the condensation stage of the highest temperature level and feed mixtures of a relatively high content of components of low boiling point are preferably fed at a point above the condensation stage of the highest temperature level.

In a further embodiment of the invention the condensation stages are provided in the form of condensers.

In a further embodiment of the invention the condensation stages are cooled by cooling water, evaporating ammonia, propane, propene and/or by exploitation of the Joule-Thompson effect when depressurising process gases.

In a further embodiment of the invention the heating device is operated by external waste heat.

The invention achieves the aim also by a distillation apparatus suitable for the separation of a feed mixture, comprising:

-   one or more inlet pipes for feeding one or more feed mixtures, -   an outlet means for the lower-boiling fraction, -   an outlet means for the higher-boiling fraction, -   at least one heating device, -   at least two successive condensers, and -   separation-effective internals installed between the condensers.

In alternative inventive embodiments of the distillation apparatus the latter consists of one single distillation column or consists of a cascade of several distillation columns in which condensers are installed between the distillation columns. In particular it may be provided that the distillation apparatus comprises 3, 4 or 5 successive condensers, each being operated at a different temperature level.

What was achieved in two process steps in the past, can now be implemented in one single process step according to the invention, without providing an upstream coldbox, which is an advantage of the invention.

By the use of the distillation apparatus according to the process involving several condensers operating at different temperature levels it is achieved that it is not required to provide the total amount of cooling fluid for the condensation at the lowest temperature level and therefore at the highest costs. Instead, the intermediate condensers are operated at temperature levels of approx. +45° C., +15° C. and −30° C., respectively, which is a further advantage of the invention.

The main part of the rising vapours in the column, if used, therefore condenses before entering the column head condenser and flows downwards as liquid. The coldness level necessary for operating the column head condenser, about −80° C., and the related condensation performance can be produced by depressurising the high-boiling substances within the apparatus itself, which is a further advantage of the invention.

In the following the invention is illustrated in detail by means of three examples:

FIG. 1 shows an embodiment of the inventive process, in which the distillation apparatus consists of one single distillation column and several intermediate condensers;

FIG. 2 shows an embodiment of the inventive process, in which the distillation apparatus comprises a stripping section and a rectifying section separately of each other;

FIG. 3 shows an embodiment of the inventive process, in which the distillation apparatus comprises three sections with intermediate condensers between these sections.

In all three examples, vaporous feed mixture 1 can be cooled down first to 15° C. in an ammonia vaporiser 2, for example. In another heat exchanger 3 the obtained C₂ fraction 4 is cooled down further to about 10° C., making part of the vapour condense. Vapour phase 5 and condensate 6 are fed separately into the distillation apparatus. Depending on the composition of the feed mixture the inlet can also be above the condensation stage of the highest temperature level.

In the example shown in FIG. 1 the liquid flows downwards in distillation column 7 and is partly evaporated again. The non-evaporated part is drawn-off as a C₃+ product 8 at the bottom of distillation column 7. The light-boiling substances rise upward as vapour and partly condense in first condenser 9, which is designed as a two-piece condenser located above the feed tray, cooling water and ammonia being used one after the other as cooling agents. Further rising vapours are partly liquefied in second condenser 10, which is operated with propane or propene as cooling agent so that only a small part of the vapours arrives at head condenser 11. The vapours not condensed in head condenser 11 constitute the C₂ fractions 12 and 13 which are depressurised after the condensation in expander 14, hereby cooling down to about −125° C. This cooled vapour 15 is used as cooling agent for the cold side of head condenser 11, where it heats up to about −50° C. Subsequently C₂ fraction 4 is conveyed through heat exchanger 3 for cooling feed mixture 1.

In the example shown in FIG. 2 the liquid flows downwards in stripping column 16 and is partly evaporated again. The non-evaporated part is drawn-off as a C₃+ product 8 at the bottom of stripping column 16. The vaporous light-boiling substances 17 flow into first condenser 9, which is designed as a two-piece condenser, where they partly condense, cooling water and ammonia being used as cooling agents one after the other. Condensate 18 and vapours 19 are given into rectification column 20. Part of the bottom product of rectification column 20 is used as reflux 21 for stripping column 16. Further rising vapours are liquefied in part in second condenser 10, which is operated with propane as cooling agent so that only a small part of the vapours arrives at head condenser 11. Vapours not being condensed in head condenser 11 constitute the C₂ fractions 12 and 13 which are depressurised after the condensation in expander 14, hereby cooling down to about −125° C. This cooled vapour 15 is used as cooling agent for the cold side of head condenser 11, where it heats up to about −50° C. Subsequently this C₂ fraction 4 is conveyed through heat exchanger 3 for cooling feed mixture 1.

In the example shown in FIG. 3 the liquid flows downwards in stripping column 16 and is partly evaporated again. The non-evaporated part is drawn-off as a C₃+ product 8 at the bottom of stripping column 16. Vaporous light-boiling substances 17 flow to first condenser 9, which is designed as a two-piece condenser, where they partly condense, cooling water and ammonia being used one after the other as cooling agents. Condensate 18 and vapours 19 are given into first rectification column 22. Part of the bottom product of first rectification column 22 is used as reflux for stripping column 16. Further rising vapours are liquefied in part in second condenser 10, which is operated with propane or propene as cooling agent, condensate and vapours are led into second rectification column 23. The bottom product of the second rectification column 23 serves as reflux for the first rectification column 22, in this way only a small part of the vapours arrives at head condenser 11. Vapours not having condensed in head condenser 11 constitute C₂ fractions 12 and 13 which are depressurised after the condensation in expander 14, hereby cooling down to about −125° C. This cooled vapour 15 is used as cooling agent for the cold side of head condenser 11, where it heats up to about −50° C. Subsequently this C₂ fraction 4 is conveyed through heat exchanger 3 for cooling feed mixture 1.

In contrast to the preceding variants, this last variant involves the advantage that only the upper part of the second rectification column must be of a design resistant against low temperature and insulated more strongly against heat transfer, which is one of the advantages of the invention.

In all three examples the heat amount necessary for the operation of the vaporisers exceeds those of the state of the art, but this heat is to be provided at only about 75° C., which is why usually waste heat from other process sections of a plant complex are used which would otherwise have to be removed expensively by means of air coolers, which is another advantage of the invention. In total, a higher refrigerating capacity will be required but the heat can be removed at a less expensive cooling level, which is another advantage of the invention.

Further advantages of the process of the invention in comparison to the processes mentioned at the beginning are:

By using cooling water, about 25% of the ammonia refrigeration capacity can be saved, so that a smaller ammonia refrigeration unit is necessary.

The propane refrigeration capacity is about 55% less so that the compressor capacity for the propane refrigeration circuit is about 50% less and the compressor can be of a correspondingly smaller size. 

1-19. (canceled)
 20. A process for the separation of a feed mixture obtained from the catalytic dehydrogenation of hydrocarbons, which contains less than 2 mol-% each of carbon dioxide and water or water vapour, into a higher-boiling and a lower-boiling fraction in a continuously operated distillation apparatus comprising at least one inlet pipe for feeding one or more feed mixtures, an outlet means for the lower boiling fraction, an outlet means for the higher boiling fraction and a heating device, comprising: providing a distillation apparatus including at least two condensation stages, each of a different temperature level; setting a higher temperature level in the condensation stages provided upstream in direction of the vapour flow than the downstream condensation stages; providing separation-effective internals between the condensation stages; wherein partial condensation takes place in the condensation stages; partial amounts that have not condensed in these stages are fed to downstream separation-effective internals or condensation stages of lower temperature level and. partial amounts that have condensed are passed via separation-effective internals in direction of the outlet means for the higher-boiling fraction; a substantially vaporous fluid is obtained at the condensation stage of the lowest temperature level, where it partially condenses; the uncondensed part of the fluid is fed to the outlet means for the lower-boiling fraction and the condensed part is passed to a section of the distillation apparatus which is upstream of the condensation stage having the lowest temperature level; and the condensation stage of the lowest temperature level has a temperature of below −40° C.
 21. The process according to claim 20, wherein the distillation apparatus comprises three to five successive condensation stages, each having a different temperature level.
 22. The process according to claim 20, wherein the condensation stage of the lowest temperature level is operated at a temperature between −120° C. and −70° C. and a pressure of at least 2 MPa absolute.
 23. The process according to claim 22, wherein the condensation stage of the lowest temperature level is operated at a temperature between −120° C. and −70° C. and a pressure of at least 3 MPa absolute.
 24. The process according to claim 20, wherein the mixture which leaves the distillation system as lower boiling fraction is depressurised, the mixture thereby cooling down by exploitation of the Joule-Thompson effect and thus being used to cool the condensation stage of the lowest temperature level.
 25. The process according to claim 24, wherein the depressurisation is performed by an expansion turbine.
 26. The process according to claim 20, wherein: the feed mixture substantially comprises hydrogen, hydrocarbons of up to two carbon atoms and hydrocarbons of at least three carbon atoms; at the outlet means for the lower-boiling fraction a mixture is obtained which contains hydrogen and hydrocarbons of up to two carbon atoms and basically no hydrocarbons of at least three carbon atoms; and at the outlet means for the higher-boiling fraction a mixture is obtained which contains hydrocarbons of at least three carbon atoms and basically neither hydrogen nor hydrocarbons of up to two carbon atoms.
 27. The process according to claim 20, wherein the segment of the distillation apparatus, via which that part of the feed mixture condensed in the condensation stage of the highest temperature level is passed to the outlet means for the higher-boiling fraction, is provided as a stripping section of the distillation apparatus.
 28. The process according to claim 20, wherein feed mixtures of a small content of components of low boiling point are preferably fed at a point below the condensation stage of the highest temperature level.
 29. The process according to claim 20, wherein feed mixtures of a high content of components of low boiling point are preferably fed at a point above the condensation stage of the highest temperature level.
 30. The process according to claim 20, wherein the condensation stages are provided in the form of condensers.
 31. The process according to claim 20, wherein the condensation stages are cooled by cooling water, ammonia, propane, propene and/or by exploitation of the Joule-Thompson effect when depressurizing the mixture leaving the distillation system as a lower-boiling fraction.
 32. The process according to claim 20, wherein one or more heating devices are operated by external waste heat.
 33. Distillation apparatus to operate a process according to claim 20, comprising: a) one or more inlet pipes for feeding one or more feed mixtures; b) an outlet means for the lower-boiling fraction; c) an outlet means for the higher-boiling fraction; d) at least one heating device; e) at least two successive condensers; and f) separation-effective internals installed between the condensers; wherein the distillation apparatus comprises a cascade of several distillation columns, in which condensers are provided between the distillation columns. 